The present invention relates to apparatus and its method of operation for cleaning fine particulate material from process and/or ventilation air filters. More particularly, the invention pertains to a vacuum cleaning system for a high efficiency particulate air (HEPA) filter operable simultaneously with the filtering mode.
High efficiency particulate air filters, commonly referred to as "HEPA" filters, are well known in the art and are widely used to filter a broad range of fine particulates (down to 0.3 micron) from air or other gases. Typical of the presently used HEPA filter construction is the CAMBRIDGE ABSOLUTE filter manufactured by the Cambridge Filter Corporation. In addition to filtering efficiencies up to 99.99% for particulates as fine as 0.3 micron, HEPA filters are characterized by their relatively simple construction and low operating pressure differential. These filters operate on the basic principle of interception of particulates from the flow of particulate-laden air through the filter media, typically a glass fiber sheet.
As is typical of mechanical interception type filters, the efficiency of a HEPA filter increases with the build-up of entrained particulates on the filter media. Correspondingly, a normal initial pressure drop of from 2 to 6" w.g. across a clean filter will also increase with the build-up of particulate material on the filter media. With a consequent sacrifice in increased power required to maintain uniform air flow, HEPA filters may be operated with increasing efficiencies up to a maximum pressure differential of about 10" w.g. At about this pressure, the filter is susceptible to rupture or other damage and resultant total filter failure.
To extend the filter life and reduce the power consumption, HEPA filters may be periodically cleaned by removing some of the entrained particulates from the filter media. Known methods of cleaning a HEPA filter include backflushing with clean air, vibrating the filter, or a combination of the two. Use of either method requires the flow of particulate air through the filter to be stopped during cleaning. In a single filter unit, interruption of the particulate air flow may require temporary process shutdown, use of an alternate filter, or the exhausting of particulate air to the atmosphere. The complications often attendant process shutdowns, the high cost of standby equipment, and the problems of air pollution make the known methods of cleaning single unit HEPA filters extremely unattractive.
In larger, multi-unit filter systems, single filter units can be isolated and periodically taken "off stream" for cleaning. However, such a system will be obviously more complex and costly than a basic multi-unit system in which the particulate air can be supplied to a simple inlet plenum feeding a number of filter units in open, parallel arrangement. In addition, if a filter must be taken off stream for periodic cleaning, the remainder of the system may have to be designed to handle the full volume particulate air flow and may, as a result, be larger and more costly.
A particularly troublesome problem exists with HEPA filters which are cleaned by vibrating or agitating the filter, especially when this method is employed without simultaneous backflushing with clean air. Within practical limits of vibrating time and at frequencies recommended by filter system manufacturers, cleaning efficiencies have been found to be quite poor and, as a result, an inordinately rapid build-up of particulates on the filter media occurs which cannot be reduced to acceptable levels. Thus, the filters must be operated at higher than necessary pressure differentials and their effective lives are substantially shortened.